If a person has good eyes, how well does he sees under water...?Can he still see good details? I mean, is it problematic to unlock a lock under water with a key or does he just have to guess where the whole is?

Your vision is greatly impaired underwater, but being able to perform task without a divemask would be highly dependent on the amount of solutes in the water.Navy divers are trained to perform task without mask, much is based on feel rather than sight. Certain waters are so murky you can't see your hand from 5 inch away, even wearing a mask, while other waters are so clear you can see quite well.What, did you make a bet or something?

Like MichaelXY said, your vision is greatly impaired underwater, even if the water itself is very clear. Unless I got this all wrong, it is because the human eye is adapted to see sharply in the air, and thus the light enters the retina optimally (or close to optimal if you need glasses) only when it comes from the air.

In water, to the contary, the refraction of light (the way light bends when it enters from one substance to another with different optical properties - here from air/water to the eye) is different from than what it is when entering from air to the eye. Hence, water animals, in general, see better in water than in the air, because their eyes have adapted to the refraction of water/eye. And thus we humans don't see much underwater.